1. Field of the Invention
The present invention relates to an acoustic structure that is able to produce a sound-absorbing effect and a sound-scattering effect, thus preventing acoustic disturbance/trouble in an acoustic space.
The present application claims priority on Japanese Patent Application No. 2011-254633 filed Nov. 22, 2011, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
It is known that an acoustic structure including a plurality of pipes (or resonance tubes) having openings at their surfaces may produce a sound-absorbing effect and a sound-scattering effect via pipes so as to prevent acoustic disturbance/trouble (e.g. flutter echo) in an acoustic space (e.g. a sound chamber). FIG. 6 shows a conventional example of an acoustic structure 90 including five pipes 25-m (where m=1 to 5) having the same length, which are horizontally aligned in a direction perpendicular to the length direction with their distal ends vertically and uniformly aligned together. Each pipe 25-m has a prismatic shape with an opening 27-m on a side face 26-m. All the pipes 25-m are equipped with the openings 27-m having the same opening area. The openings 27-m are formed at different positions on the side faces 26-m of the pipes 25-m along with the length direction.
The acoustic structure 90 may produce a sound-absorbing effect and a sound-scattering effect based on a certain acoustic principle, which will be described with reference to FIG. 7. FIG. 7 shows a cross section of a pipe 25-m with an opening 27-m on a side face 26-m. It is possible to construe that the back cavity of the opening 27-m of the pipe 25-m is divided into a closed pipe portion CPL with a closed end 28L-m and an open end using the opening 27-m, and another closed pipe portion CPR with a closed end 28R-m and an open end using the opening 27-m. With sound waves entering into the cavity via the opening 27-m from an acoustic space, progressive waves may travel in the leftward direction from the open end (i.e. the opening 27-m) to the closed end 28L-m of the closed pipe portion CPL while progressive waves may travel in the rightward direction from the open end (i.e. the opening 27-m) to the closed end 28R-m of the closed pipe portion CPR. Leftward progressive waves are reflected on the closed end 28L-m of the closed pipe portion CPL so that reflected waves may be retransmitted to the opening 27-m, while rightward progressive waves are reflected on the closed end 28R-m of the closed pipe portion CPR so that reflected waves may be retransmitted to the opening 27-m. 
The closed pipe portion CPL causes resonance at a resonance frequency fL-n (where n=1, 2, . . . ) according to Equation (1). The closed pipe portion CPL combines progressive waves and reflected waves to produce standing waves with a node at the closed end 28L-m and an antinode at the open end depending on particle velocity. Additionally, the closed pipe portion CPR causes resonance at a resonance frequency fR-n (where n=1, 2, . . . ) according to Equation (2). The closed pipe portion CPR combines progressive waves and reflected waves to produce standing waves with a node at the closed end 28R-m and an antinode at the open end depending on particle velocity. In Equations (1), (2), LL denotes the length of the closed pipe portion CPL (i.e. the length measured between the left-side closed end 28L-m to the opening 27-m); LR denotes the length of the closed pipe portion CPR (i.e. the length measured between the right-side closed end 28R-m to the opening 27-m); c denotes propagation speed of sound waves; and n is an integer equal to or greater than “1”.fL-n=(2n−1)−(c/(4-LL))  (1)fR-n=(2n−1)−(c/(4-LR))  (2)
Sound waves of the resonance frequency fL-n may reach the periphery of the opening 27-m of the side face 26-m while partially entering into the opening 27-m of the pipe 25-m. Herein sound waves are reflected on the closed end 28L-m of the closed pipe portion CPL and then emitted from the opening 27-m toward an acoustic space. They have a reverse phase compared to the phase of sound waves entering into the opening 27-m from an acoustic space. Sound waves originated in an acoustic space are reflected on the periphery of the opening 27-m of the side face 26-m of the pipe 25-m without being involved in phase rotation.
With sound waves of the resonance frequency fL-n entering into the cavity via the opening 27-m from an acoustic space, the closed pipe portion CPL may produce a sound-absorbing effect because sound waves incoming in the normal direction (or the front direction) of the opening 27-m of the side face 26-m interfere with reverse-phase sound waves, i.e. sound waves emitted out of the opening 27-m and sound waves reflected on the periphery of the opening 27-m of the side face 26-m. Additionally, a certain flow of air molecules may occur to cancel out phase discontinuity between sound waves emitted out of the opening 27-m and sound waves reflected on the periphery of the opening 27-m of the side face 26-m. This may cause a sound-scattering effect due to a flow of acoustic energy in any direction, other than the reflected direction opposite to the incoming direction of sound waves, in the periphery of the opening 27-m of the side face 26-m. 
With sound waves of the resonance frequency fR-n entering into the cavity via the opening 27-m from an acoustic space, the closed pipe portion CPR may produce a sound-absorbing effect in the normal direction (or the front direction) of the opening 27-m of the side face 26-m. Additionally, the closed pipe portion CPR may produce a sound-scattering effect in the periphery of the opening 27-m of the side face 26-m. As described above, each of the closed pipe portions CPL and CPR produces a sound-absorbing effect and a sound-scattering effect based on the above acoustic principle.
Patent Literatures 1-3 disclose an acoustic structure which may operate based on the above acoustic principle. This acoustic structure is able to improve a sound-absorbing effect and a sound-scattering effect with a reduced area SO of an opening smaller than a sectional area SP of a cavity of a pipe. With sound waves entering into the cavity of the pipe from the opening of the acoustic structure, the behavior of a medium at the opening may depend on a specific acoustic impedance ratio ζ at the opening. The specific acoustic impedance ratio ζ is a complex ratio of an acoustic impedance ratio ZA at a certain point in a sound field to a characteristic impedance ratio ZC of a medium at a certain point. The specific acoustic impedance ratio ζ at a certain point of the opening 27-m receiving sound waves at a certain frequency is given by Equation (3), where j denotes an imaginary unit; LL denotes the length of the closed pipe portion CPL; LR denotes the length of the closed pipe portion CPR; and k denotes a wave number (specifically, a value of 2πf/c which is produced by diving angular velocity 2πf of an incoming wave by sound speed c, where f denotes frequency).
                    ζ        =                                            Z              A                                      Z              C                                =                                    -              j                        ⁢                                          S                O                                            S                P                                      ×                                          cos                ⁢                                                                  ⁢                                  kL                  L                                ×                cos                ⁢                                                                  ⁢                                  kL                  R                                                            sin                ⁢                                                                  ⁢                                  k                  ⁡                                      (                                                                  L                        L                                            +                                              L                        R                                                              )                                                                                                          (        3        )            
FIG. 8 shows the known relationship between the phase of an incoming wave and the phase of a reflected wave on the opening of a pipe included in an acoustic structure. When an absolute value |Im(ζ)∥ of a specific acoustic impedance ratio ζ at the interface of a medium is zero, a phase difference φ between an incoming wave at the interface of a medium and a reflected wave reflected on the interface of a medium may be equal to ±180° (i.e. their phases are reverse to each other). In the range of |m(ζ)|<1 (i.e. a graphic region outside a semicircular region with hatching on a Gaussian plane shown in FIG. 8 in which Im(ζ) denotes an imaginary part, and Re(ζ) denotes a real part with respect to a specific acoustic impedance ratio ζ), the phase difference φ becomes smaller than ±90°. For this reason, the acoustic structure of FIG. 6 may maximize a sound-absorbing effect and a sound-scattering effect when the absolute value |Im(ζ)| of the imaginary part Im(ζ) of the specific acoustic impedance ratio ζ on the opening of a pipe becomes zero. In contrast, the acoustic structure may not produce a sound-absorbing effect and a sound-scattering effect substantially when the absolute value |Im(ζ)| exceeds “1”. According to Equation (3) stipulating the relationship between the specific acoustic impedance ratio ζ and the ratio SO/SP (i.e. the ratio of the area SO of the opening of a pipe to the sectional area SP of the cavity), it is possible to increase the bandwidth of a band causing a sound-absorbing effect and a sound-scattering effect close to the resonance frequencies fL-n, fR-n (i.e. a band in which the absolute value |Im(ζ)| of the imaginary part Im(ζ) of the specific acoustic impedance ratio ζ becomes less than “1”) in response to the ratio SO/SP getting smaller. FIG. 9 shows the result of experiments which the inventor of this application (i.e. the inventor of Patent Literature 1) conducted to evaluate the foregoing behavior of a pipe in an acoustic structure. In experiments, the inventor of this application attempted to evaluate the behavior of a single pipe with dimensions described in Table relating to the length LL of the closed pipe portion CPL, the length LR of the closed pipe portion CPR, and the ratio SO/SP between the area SO of the opening and the sectional area SP of the cavity perpendicular to the opening, thus calculating the absolute value |Im(ζ)| of the imaginary part Im(ζ) of the specific acoustic impedance ratio at frequencies ranging from 0 Hz to 1,000 Hz.
TABLELL (mm)LR (mm)SO/SPGraph3004850.25Solid Line3004851Dashed Line3004854Dotted Line
Through comparison between three examples with SO/SP=0.25, SO/Sp=1, SO/SP=4 in terms of the bandwidth of a band corresponding to I Im(ζ)| less than “1”, an example of SO/SP=0.25 indicates the largest bandwidth while an example with SO/SP=4 indicates the smallest bandwidth. Compared to a pipe with a large ratio SO/SP, a pipe with a small ratio SO/SP is able to produce a sound-absorbing effect and a sound-scattering effect in a wide band. This proves an insight that an acoustic structure including a pipe with the opening area SO smaller than the sectional area SP of the cavity is able to improve a sound-absorbing effect and a sound-scattering effect.
As disclosed in Patent Literature 1, the bandwidth of a band causing a sound-absorbing effect and a sound-scattering effect with a pipe of an acoustic structure depends on the ratio between the opening area and the sectional area of the cavity of a pipe. However, the conventional acoustic structure is unable to change the ratio between the opening area and the sectional area of the cavity of a pipe; hence, it is impossible to adjust the bandwidth of a band causing a sound-absorbing effect and a sound-scattering effect in an acoustic space (e.g. a sound chamber) equipped with an acoustic structure.